Which one of the following equations represents a one-dimensional wave equation?

1. $\dfrac{\partial u}{\partial t}=C^{2}\dfrac{\partial ^{2}u}{\partial x^{2}} \\$
2. $\dfrac{\partial ^{2}u}{\partial t^{2}}=C^{2}\dfrac{\partial ^{2}u}{\partial x^{2}} \\$
3. $\dfrac{\partial ^{2}u}{\partial t^{2}}=C^{2}\dfrac{\partial u}{\partial x} \\$
4. $\dfrac{\partial ^{2}u}{\partial t^{2}}+\dfrac{\partial ^{2}u}{\partial x^{2}}=0$